1. Field of the Invention
This invention is directed to vanadium oxide cathode mixtures comprising a mixture of a vanadium oxide cathode material and an amount of one or more surface active agents. This invention is further directed to electrochemical cells containing as the active cathode material a mixture of at least one vanadium oxide and at least one surface active agent.
This invention is also directed to methods for enhancing the cumulative capacity and cycling capability of electrochemical cells containing vanadium oxides as the active cathode material by employing as the cathode material a mixture containing at least one vanadium oxide and at least one surface active agent.
2. State of the Art
The use of vanadium oxides as the active cathode materials in electrochemical cells, in particular nonaqueous primary and secondary cells, is well known in the art.
For example, the use of V.sub.2 O.sub.5 as a cathode material in a primary cell is described in Dey et al, U.S. Pat. No. 3,655,585. Moreover, the use of V.sub.2 O.sub.5 in a secondary cell was first reported by Walk and Gore, Electrochemical Society Meeting, Paper No. 27, Toronto, May 11-16 (1975).
However, secondary cells containing V.sub.2 O.sub.5 as the active cathode material suffer from various drawbacks, e.g., the low electronic conductivity of V.sub.2 O.sub.5 and its irreversible reduction at moderate potentials. Additionally, electrolyte oxidation occurs during charging of V.sub.2 O.sub.5 containing cells.
Accordingly, it is further known to use other vanadium oxides, as an alternative to V.sub.2 O.sub.5, as the active cathode material in electrochemical cells. For example, Christian et al, U.S. Pat. No. 4,228,226 describe nonaqueous cells comprising lithium metal as the active anode material and vanadium oxides having the normal stoichiometry VO.sub.2+y, wherein y is greater than or equal to approximately 0.4. Such vanadium oxides include e.g., VO.sub.2, V.sub.3 O.sub.7, V.sub.4 O.sub.9 and V.sub.6 O.sub.13.
It is also known in the art to use vanadium oxide mixtures which contain other materials, e.g. conductive diluents such as carbonaceous materials, as the active cathode material in electrochemical cells.
For example, Uchiyama et al, U.S. Pat. No. 4,751,197, describes a cathode for use in a lithium electrochemical cell comprising a mix of a mixed metal-oxide prepared from V.sub.2 O.sub.5 and MoO.sub.3, a conductive diluent, and an aqueous based binder wherein the mixture is rolled onto a nickel screen and sintered under vacuum at about 280.degree. C.
Additionally, Hope et al, U.S. Pat. No. 4,576,886, describes a solidstate lithium battery wherein the cathode comprises a layer of polymer spheres, wherein each polymer sphere consists of a vanadium oxide core encapsulated in an ionically and electronically conductive polymeric material. Hope et al indicate that such a composite cathode construction dramatically improves the performance characteristics of lithium anode based solid-state cells.
Related to the Hope et al patent, Rourke et al, U.S. Pat. No. 4,720,410 describes a method for preparing composite cathodes comprising insertion materials including, e.g., vanadium oxides, wherein said insertion compound, in particular vanadium oxide, and an inorganic salt are dispersed in a solution of a polymer contained in a volatile solvent, and then spray dried to remove the solvent and produce cathode spheres comprising the insertion compound as a core material encapsulated in a polymeric matrix containing the inorganic salt.
Additionally, Buchel et al, U.S. Pat. No. 4,952,467, describe a process for producing a composite cathode composition containing a vanadium oxide comprising preparing a powdered mixture of a vanadium oxide (vanadium pentoxide), a carbon, and a mixture of alkaline halides, and then heat processing at a temperature ranging from 360.degree. C. to 650.degree. C. for a time ranging from about 15 minutes to about 2 hours.
It is also known in the art to use surface active agents to improve the performance of electrochemical cells. For example, Voorhles et al, U.S. Pat. No. 3,634,138 teach improving the shelf life and cycle life of secondary cells containing zinc anodes, and an azobisformamide or substituted azobisformamide depolarizer by coating the zinc anode with a small amount of a tetraalkylammonium salt, and by increasing the content of ammonium chloride in the electrolyte mixture.
Additionally, Broadhead et al, U.S. Pat. No. 3,928,067, describes a lithium nonaqueous secondary battery containing dopants which improve the recycling characteristics of said battery by acting as wetting agents for a polypropylene separator contained therein.
Further, Schmidt, U.S. Pat. No. 4,440,838, describes an improved battery separator for lead-acid battery cells which is formed by depositing an improved wetting composition onto a polyolefin substrate comprised of entangled microfibers, wherein the wetting composition comprises an epoxy compound, a wetting agent and an emulsifier.
Also, Anderman et al, U.S. Pat. Nos. 4,654,281, 4,853,305, 4,735,875 and 5,143,805, describe cathode compositions comprising a microporous sheet composed of from 2-30 weight percent polyethylene, 70-98 weight percent of electrically conductive and electrochemically active particulate material, and from 0 to 5.0 weight percent of a plasticizer for the polyethylene. In addition to the above described components, Anderman et al suggest that the admixture may contain conventional stabilizers, antioxidants, wetting agents, processing acids or mixtures thereof, with sodium alkyl benzene sulfonate, sodium lauryl sulfate, dioctyl sodium sulfosuccinate and isooctyl phenyl polyethoxy ethanol being exemplary of known commercially available wetting agents. However, while vanadium oxides are among the numerous identified cathode materials named by Anderman et al, these patents fail to describe any specific cathode mixtures comprising a vanadium oxide in combination with a surface active agent.
Notwithstanding the above, the capacity (cathode utilization) and cycle life of electrochemical cells, in particular solid batteries, containing vanadium oxides as the active cathode material is often less than desirable. Moreover, even when the initial capacity of the solid batteries is relatively high, the solid batteries often exhibit rapid decline in capacity over their cycle life.
Specifically, the cumulative capacity of an electrochemical cell, e.g., a solid battery, is the summation of the capacity of the solid battery over each cycle (charge and discharge) in a specified cycle life. Solid batteries having a high initial capacity but which rapidly lose capacity over the cycle life will have low cumulative capacity which interferes with the effectiveness of these batteries for prolonged usage.
In view of the above, the art is searching for methods to improve the capacity and cycle life of electrochemical cells, in particular solid batteries, which contain vanadium oxides as the active cathode material. It goes without saying that increases in capacity and cycle life of electrochemical cells comprising vanadium oxides as the active cathode material would enhance their widespread commercial usage.